Building a Two-Node Linux Cluster with Heartbeat

The term "cluster" is actually not very
well defined and could mean different things to different people.
According to
Webopedia, cluster
refers to a group of disk sectors. Most Windows users are probably
familiar with lost clusters--something that can be rectified by
running the defrag utility.

However, at a more advanced level in the computer industry,
cluster usually refers to a group of computers connected together
so that more computer power, e.g., more MIPS (millions instruction
per second), can be achieved or higher availability (HA) can be
obtained.

Beowulf, Super Computer for the "Poor"
Approach

Most super computers in the world are built on the concept of
parallel processing--high-speed computer power is achieved by
pulling the power from each individual computer. Made by IBM, "Deep
Blue", the super computer that played chess with the world champion
Garry Kasprov, was a computer cluster that consisted of several
hundreds of RS6000s. In fact, many big time Hollywood movie
animation companies, such as Pixar, Industrial Light and Magic, use
computer clusters extensively for rendering (a process to translate
all the information such as color, movement, physical properties,
etc., into a single frame of picture).

In the past, a super computer was an expensive deluxe item
that only few universities or research centers could afford.
Started at NASA, Beowulf is a project of building clusters with
"off-the-shelf" hardware (e.g., Pentium PCs) running Linux at a
very low cost.

In the last several years, many universities world-wide have
set up Beowulf clusters for the purpose of scientific research or
simply for exploration of the frontier of super computer
building.

High Availability (HA) Cluster

Clusters in this category use various technologies to gain an
extra level of reliability for a service. Companies such as Red
Hat, TurboLinux and PolyServe have cluster products that would
allow a group of computers to monitor each other; when a master
server (e.g., a web server) goes down, a secondary server will take
over the services, similar to "disk mirroring" among
servers.

Simple Theory

Because I do not have access to more than one real (or
public) IP address, I set up my two-node cluster in a private
network environment with some Linux servers and some Win9x
workstations.

If you have access to three or more real/public IP addresses,
you can certainly set up the Linux cluster with real IP
addresses.

In the above network diagram (fig1.gif), the Linux router is
the gateway to the Internet, and it consists of two IP addresses.
The real IP, 24.32.114.35, is attached to a network card (eth1) in
the Linux router and should be connected to either an ADSL modem or
a cable modem for internet access.

The two-node Linux router consists of node1 (192.168.1.2) and
node2 (192.168.1.3). Depending on your setup, either node1 or node2
can be your primary server, and the other will be your backup
server. In this example, I will choose node1 as my primary and
node2 as my backup. Once the cluster is set, with IP aliasing (read
IP aliasing from the Linux Mini HOWTO for more detail), the primary
server will be running with an extra IP address (192.168.1.4). As
long as the primary server is up and running, services (e.g., DHCP,
DNS, HTTP, FTP, etc.) on node1 can be accessed by either
192.168.1.2 or 192.168.1.4. In fact, IP aliasing is the key concept
for setting up this two-node Linux cluster.

When node1 (the primary server) goes down, node2 will be take
over all services from node1 by starting the same IP alias
(192.168.1.4) and all subsequent services. In fact, some services
can co-exist between node1 and node2 (e.g., FTP, HTTP, Samba,
etc.), however, a service such as DCHP can have only one single
running copy on the same physical segment. Likewise, we can never
have two identical IP addresses running on two different nodes in
the same network.

In fact, the underlining principle of a two-node,
high-availability cluster is quite simple, and people with some
basic shell programming techniques could probably write a shell
script to build the cluster. We can set up an infinite loop within
which the backup server (node2) simply keeps pinging the primary
server, if the result is unsuccessful, and then start the floating
IP (192.168.1.4) as well as the necessary dæmons (programs
running at the background).

A Two-Node Linux Cluster HOWTO with
"Heartbeat"

You need two Pentium class PCs with a minimum specification
of a 100MHz CPU, 32MB RAM, one NIC (network interface card), 1G
hard drive. The two PCs need not be identical. In my experiment, I
used an AMD K6 350M Hz and a Pentium 200 MMX. I chose the AMD as my
primary server as it can complete a reboot (you need to do a few
reboots for testing) faster than the Pentium 200. With the great
support of CFSL (Computers for Schools and Libraries) in Winnipeg,
I got some 4GB SCSI hard drives as well as some Adaptec 2940 PCI
SCSI controllers. The old and almost obsolete equipment is in good
working condition and is perfect for this experiment.

Comments

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I have never even heard of running defrag for a windows disk problem and no one I know has ever run defrag for that purpose. Defrag is for consolidating data not fixing errors. Why would you even suggest doing this???? Scandisk is the standard tool that everyone uses and runs automatically most times you have a lockup or improper shutdown.

Please for anyone reading the above message do NOT run defrag if you are getting disk errors. Run scandisk. If you continue to have problems your disk may be going bad.

ok heartbeat works great - really appreciated ! and
i am amazed the way it worked perfectly.
how do we mount the san partition for that IP service in active-active apache cluster.
do i make an entry in /etc/fstab or is that a sin ?
I mea i need to mount /var/www/html on /dev/sdc1 . so i do i mak a entry in fstab file ?

ok heartbeat works great - really appreciated ! and
i am amazed the way it worked perfectly.
how do we mount the san partition for that IP service in active-active apache cluster.
do i make an entry in /etc/fstab or is that a sin ?